Technically, 3rd person pronouns do not exist in Latin as they do in English. However, they do have equivalents.

Adjectives modify nouns and take the gender of the noun which they modify. However, adjectives do not necessarily need a substantive present in the sentence to modify. The substantive can be presumed. In this way, '3rd person' pronouns are formed.

Take the masculine form of the adjective 'ille'. Literally it means 'That (masculine) thing.' However one could take it for simply meaning 'he', depending on the context. Similarly, the pronoun 'iste' means 'this (masc.) thing'. Iste and ille are declined in exactly the same way.

The relative pronoun takes on the case depending on the function it serves in the relative clause. For example, in the sentence "He sees the man who has a slave," "who" is translated as nominative because it is the subject of the clause "who has a slave." The antecedent (noun to which the pronoun refers) is usually before the relative clause.